Gas-generating grain



April 16, 1968 G. VAN DEENE EMERSON 3,377,956

GAS -GENERATING GRAIN Filed Oct. 22, 1965 hu INVENTOR L5 GERA/e0 u EMERSON AGENT United States Patent O 3,377,956 GAS-GENERATENG GRAIN Gerard Van leene Emerson, Fairfax, Va., assigner to rlille Susquehanna Corperation, a corporation of Delaware Filed Oct. 22, 1965, Ser. No. G2,021 11 Claims. (Cl. 162-102) ABSTRACT 0F THE DISCLOSURE In a propellant grain having a singlelongitudinally embedded metal heat conductor, a plurality of second heat conductors are embedded in the head end so that they subscribe a truncated cone-shaped envelope converging toward the ignition end of the grain. This arrangement steers the llame front during the last portion of buring so that the buring area is kept at a maximum for as long as possible, and the maximum amount of propellant is burned.

This invention relates to a new and improved endburning gas-generating or propellant grain.` Particularly, the invention relates to an end-burning gas-generating or propellant grain containing a single longitudinally-embedded metal heat conductor having improved rapid tailoif characteristics.

For purposes of this invention the term gas-generating or propellant grain means a shaped charge comprising a matrix composition -Which is self-oxidant, namely capable of self sustaining combustion to generate hightemperature gases, such as CO, CO2, H2, H2O, etc. Such gases can be used for any suitable and well-known purpose, such as the generation of power or thrust as in driving a turbine or a reaction motor, the testing of high temperature insulation materials, or the production'of gases for attitude control, and thelike. The gas-generating grain matrix 'can be any conventional propellant composition. It can be, for example, of the double base type such as nitrocellulose gelatinized with nitroglycerine, or of the composite type, such as a mixture of an organic fuel, e.g. a polymer such as polyvinyl chloride, polysulde, polybutadiene, etc., and a finely-divided inorganic oxidizer, e.g. ammonium perchlorate or ammonium nitrate. It can include additives such as metal fuel powders, chopped metal wires or staples, burning rate modifiers, and the like.

End-burning grains containing elongated metal heat conductors, such as wires, which are embedded in intimate, gas-sealing contact with the propellant matrix, positioned normal to the initial ignition surface of the grain, and continuously disposed in the -direction of ame prop- -agation of the grain, are known to the art. See, for example, U.S. Patent 3,140,663. Suoh grains, referred to herein as wired grains, have eliminated the disadvantages of prior end-burning grains by greatly increasing burning surface area and, thereby, the effective mass burning rate and mass rate of gas generation to the degree requisite for high performance. The large burning surface area of the wired end-burning grains results from the fact that the matrix burns at an exceedingly rapid rate along the metal heat conductor thereby producing recessing of the burning surface, with the Wire at the apex of the formed recess. Thus wired end-burning grains can now be employed in place of grains having longitudinal perforations or lateral recesses, expedients which are disadvantageous since they reduce grain strength and motor loading capacity.

Such grains in practice have required the use of a large plurality of laterally-spaced, longitudinally-embedded continuous wires, in some cases as many as 50 or more, in order to obtain rapid attainment of equilibrium burning and, particularly, rapid tail-off with minimum formation of slivers. The reasons for this are explained in the above-referenced U.S. Patent 3,140,663.

The use of a plurality of wires has certain disadvantages, Embedding the many tine wires in correct position and without breaking is a rather slow, diiicult, and costly processing step.

However, use of a single wire has posed severe problems of long tail-off because the recessed cone formed along a single wire continues to deepen and widen until it encompasses the entire width of the grain. When burning has progressed along the full length of the single wire, the large remaining forward or head-end portion of the grain surrounding the conical burning surface must then burn out at the normal linear burning rate of the propellant matrix. Not only is the high mass burning rate induced by the metal wire no longer available but the large conically-recessed burning surface results in a continuously regressive burning surface area. The net effect is that the rate of gas generation regresses over a long tail-off period.

The object of the invention is to provide an end-burning grain containing a single, longitudinally-embedded metal heat conductor, which is characterized by rapid tail-off.

Other objects and advantages will become obvious from the following detailed description and the drawings, in which like parts are identified by the same reference character:

FIGURE l is a longitudinal sectional view through a single-wired gas-generating grain according to the invention and the effect on surface area of the grain during burning;

FIGURE 2 is a cross-sectional yiew taken along line 2-2 of FIGURE l;

FIGURES 3 and 4 are perspective views of modica' tions of hollow metal truncated conical heat conductor elements that can be employed in other embodiments of this invention.

I have discovered that the desired rapid tail-off can be obtained in an end-burning grain containing a single longitudinally embedded axial metal heating conductor, preferably a Wire, by embedding a tubular metal heat conductor in the forward or head end portion only of the grain in such manner that its longitudinal axis corresponds with the longitudinal axis of the grain, with its base preferably terminating substantially at the head end of the grain. The tubular metal heat conductor can have longitudinal, axially-oriented sides. For maximum ei`n`ciency in reducing tail-off, the sides are preferably convergent toward the ignition end of the grain. In cross-section, the tubular, convergently-sided heat conductor can be of any suitable shape, e.g. circular, oval, or polygonal. Preferably, it is an open-ended, hollow truncated, conical element. For purpose of convenience, such a conical element will be used in the following detailed description of the invention. Preferably, although this is not essential, the longitudinally-embedded wire extends the full length of the grain and along the longitudinal axis of the hollow metal truncated conical member.

The hollow truncated metal cone provides a substantially annular heat conducting ele-ment in the peripheral head-end portion of the grain surrounding the conical burning surface produced by the axially-embedded longitudinal wire. When the conical burning surface produced by the wire intersects the aft end of the truncated metal cone, thc burning surface begins to invert `along the metal cone to produce an annular trough-like depression of V- shaped cross-section in at least one transverse dimension with the conical metal member intersecting the apex. Thus the desired large burning surface area is maintained substantially to burn-out of the grain without the regression that would otherwise occur when burning has reached the head-end termination of the axially-embedded wire. If the metal species and wall thickness of the truncated metal cone are selected to produce the same equilibrium apex angle as that of the conica-l burning surface surrounding the wire, the total equilibrium burning surface remains substantially constant substantially to burn-out. Tail-off is sharp and sliver formation is substantially eliminated.

FIGURE 1 illustrates diagrammatically the burning phenomenon that occurs during combustion of an endburning propellant grain 2 having longitudinally embedded in it, normal to the plane of initial ignition surface 3 single metal wire 4. The end-burning grain is inhibited on its lateral and forward or head-end surfaces by inhibitor coating 5. Embedded longitudinally and coaxially in the forward portion of the grain is hollow metal truncated cone 6 seated so that its base terminates at the forward end 7 of the grain.

Grain 2 as shown is provided with plane ignition surface 3 so that attainment of the equilibrium burning surface after initial ignition or start-up is relatively slow and progressive. It will be understood that modifications of the aft end of the grain to increase the rate of attainment of equilibrium burning as shown in U.S. Patent 3,140,663 or in William H. Holter et al. application, Gas-Generating Grains, filed concurrently herewith, can be employed herein.

After ignition of surface 3, metal wire 4 is heated by the hot combustion products and the burning rate of the grain matrix adjacent to the hot wire increases to cause recessing into the shape of an inverted cone with the wire at the apex. As shown, when burning has reached 3a, conical recess Sa is shallow. lt continues to deepen as shown at burning surface 3b and cone 8b until it intersects the peripheral sides of the grain and has reached its equilibriu-rn burning angle a. Conical burning surface 8c is the equilibrium burning surface and remains constant until the apex of conical burning surface 8c' reach-es the end A of the wire. In the absence of embedded hollow truncated metal cone 6, the forward portion of the grain defined in longitudinal section by BACDEB continues to burn regressively at the normal linear burning rate of the grain matrix with corresponding long tail-off.

When inverted conical burning surface 8c intersects aft end 9 of truncated metal cone 6, the burning surface recesses along the wall of the metal cone forming annular trough 10 (shown when burning has reached the head-end base of the truncated nretal corre) of V-shaped crosssection arrd apex angle a. It is thus seen that the burning surface is maintained at the desired large 4area by the truncated metal cone substantially until burn-out of the grain with concomitant rapid tail-off.

The burning surface area and mass burning rate for a particular grain are determined by such factors as the linear burning rate of the grain matrix composition, the metal species of the metal heat conductors, their crosssectional dimensions, and the coating on the conductors, if any. When the above factors have been determined, the shape, size and metal species of the truncated metal cone can be varied as desired to produce a tail-off curve of any desired slope.

As aforementioned, the axial wire preferably extends the full length of the grain which includes the longitudinal axis of the coaxially-embedded truncated metal cone. The wire can, if desired, terminate aft of the head-end base of the truncated cone. It should, however, extend far enough along the longitudinal axis of the truncated cone to provide a recessed conical burning surface that intersects with the aft end of the truncated cone.

The axial, longitudinally embedded wire can be of any suitable cross sectional shape, eg. circular, rectangular, oval, or the like. It can also be ya at, narrow strip, which as employed herein can be considered a flat wire. The maximum thickness of the wire in at least one transverse i direction should be about 0.05 inch, preferably a maximum of about 0.02 inch. Optimum thickness is generally about 0.003 to 0.01 inch.

The tubular metal heat conductor, such as the hollow truncated metal cone can have a solid, unperfor'ated wall. It can also be perforated, as in the form of screening, or comprise longitudinal wires (including narrow strips) arranged to form a tubular pattern, preferably convergent, as shown in FIGURES 3 and 4. In FIGURE 3 hollow metal truncated conical heat conductor 20 comprises perforated wail 21 made of wire mesh or screen. In FIG- URE 4 the conical element 30 comprises a perforated wall consisting of wires 31 attached at the apical surface or narrower aft end to transverse ring 32 and at the base to transverse ring 33. The unper'forated or perforated sheet or wire elements forming the hollow truncated metal cone should, like the axial longitudinally embedded single wire, have a maximum thickness of about 0.05 inch, preferably a maximum of about 0.02 inch.

The metal heat conductor forming the axial longitudinally embedded wire and the coaxial embedded truncated corre are preferably siiver, copper, or aluminum, although they can be any other metal or metal alloy having good heat conductive properties, such as platinum, tungsten, magnesium molybdenum, steel, and the like. The wire and the tubular member can be of the same or different metal species.

The metal heat conductors can comprise a core of one metal species, such as silver, coated with a different metal species, such as a metal of higher melting point, eg., tungsten.

The metal heat conductors can be coated with an inert insulator type coating to tailor the mass burning rate to a level between that of the propellant matrix and the burning rate induced by `a bare metal conductor of the same metal species and size. Such coatings and their effect are described in U.S. 3,109,374.

The metal heat conductors can also be coated with self-oxidant coatings of lower or higher linear burning rates than that of 4the propellant grain matrix to reduce or increase the mass burning rate produced by the bare metal conductor of the same metal species and size. Such Coatings and their effect are described in U.S. 3,109,375.

As aforementioned, the grain matrix can be any conventional solid self-oxidant, gas-generating composition. It can also be a plastic semi-solid. Colresive, shape-retentive, monopropellant compositions, which are characterized as plastic or semi-solid because they flow at ambient or normal temperatures under moderate stress or pressure, can be loaded into the combustion chamber of a gas-generating device, where they function as endburning grains. Examples of semi-solid monopropellant compositions suitable for such use are disclosed in U.S. 3,095,334, 3,107,187, 3,113,894, and 3,196,059.

Although this invention has been described with reference to an illustrative embodiment thereof, it will be apparent to those skilled in the art that the principles of the invention can be embodied in other forms within the scope of the claims.

I claim:

1. In an end-burning grain having an initial ignition surface at one end and a restricted head end surface at its opposite end and comprising a self-oxidant gas-gerrerating matrix having a single elongated metal wire or narrow strip longitudinally and axially embedded in the matrix of said grain normal to the initial ignition surface, the improvement comprising a tubular metal heat conductor longitudinally embedded in the end portion of said grain coaxially with said wire or strip, the sides of said tubular heat conductor being convergent toward the initial ignition surface of said grain.

2. The grain of claim 1 in which the tubular corrvergently-sided heat conductor is a hollow truncated conical member, the base of which terminates adjacent to the head-end surface of said grain.

3. The grain of claim 1 in which the metal wire or strip extends the full length of said grain.

4. The grain of claim 1 in which the convergent sides of the tubular heat conductor are perforated.

5. The grain of claim 4 in which the convergent sides comprise a plurality of the longitudinally-extending wires or strips.

6. The grain of claim 4 in which the convergent sides of the tubular heat conductor comprise metal screen.

7. The grain of claim 2 in which the metal Wire or strip extends the full length of said grain.

8. The grain of claim 2 in which the Wall of the lconical heat conductor is perforated.

9. The grain of claim 2 in which the conical heat con- References Cited UNITED STATES PATENTS 7/ 1964 Rumbel et al. 102-98 ROBERT F. STAHL, Primary Examiner.

SAMUEL W. ENGLE, Examiner. 

